This invention relates to distillates materials having high cetane number and useful as a diesel fuel with sulfur-free, nitrogen-free or aromatics-free, as well as the process for preparing the diesel distillates. More particularly, this invention relates to a process for preparing directly diesel distillates from synthesis gas over modified Fischer-Tropsch catalysts that cut off the heavier end of the Schultz-Flory distribution.
Oil fields typically have deposits of natural gas associated with them. In remote locations where transport of this gas may not be economically attractive, gas conversion technology can be used for chemically converting natural gas to higher molecular weight hydrocarbons. Current gas conversion technologies rely on the chemical conversion of natural gas to synthesis gas, which is a mixture of carbon monoxide and hydrogen. Synthesis gas is then reacted in a catalyzed hydrocarbon synthesis process commonly known as Fischer-Tropsch synthesis.
Fischer-Tropsch reaction dates back to 1923, with the discovery of an efficient catalyst to convert synthesis gas into hydrocarbons mixtures. After coal-based synthetic fuels production during the World War II in Germany, and later in South Africa (SASOL), the energy crisis of 70""s and 80""s renewed the interest toward the conversion of the increasing remote natural gas reserves to liquid fuel (GTL).
Clean diesel distillates that contain no or nil sulfur, nitrogen, or aromatics, are, or will likely be in great demand as diesel fuel or in blending diesel fuel. Clean diesel distillates having relatively high cetane number are particularly valuable. Typical petroleum derived distillates are not clean, in that they typically contain significant amounts of sulfur, nitrogen, and aromatics, and they have relatively low cetane numbers. Clean diesel distillates can be produced from petroleum based distillates through severe hydrotreating at great expense. Such severe hydrotreating imparts relatively little improvement in cetane number and also adversely impacts the fuel""s lubricity. Fuel lubricity, required for the efficient operation of fuel delivery system, can be improved by the use of costly additive packages.
U.S. Pat. No. 4,478,954 discloses a catalyst for the production of C2xcx9cC4 olefins from carbon monoxide and hydrogen consisting of 5xcx9c50 wt % iron metal and 20xcx9c90 wt % an activated carbon support, promoted by magnesium and chromium metals.
U.S. Pat. No. 4,542,122 states that a cobalt catalyst, especially a thoria promoted cobalt catalyst, formed by dispersing the cobalt, or cobalt and thoria, upon a titania or titania-containing support wherein the titania support is one having a rutile:anatase ratio of at least about 2:3. By passing methanol, or a mixture of carbon monoxide and hydrogen over the catalyst at reaction conditions, a distillate fuel consisted principally of a mixture of linear paraffin and olefins, particularly a C10+ distillate can be formed. The distillate can be further refined and upgraded to high quality fuels, and other products such as diesel fuel, jet fuel, lubes and specialty solvents, particularly premium middle distillate fuels of carbon numbers ranging from about C10 to about C20.
U.S. Pat. No. 4,579,986 discloses that synthesis gas is subjected to Fischer-Tropsch synthesis over a Co/Zr/SiO2 catalyst and the C20+ fraction of the synthesized product is converted into linear C10-C20 olefins by mild thermal cracking.
Venter and Vannice (J. Catal., 1987, 103:450; Catal. Lett., 1990, 7:219; J. Phys. Chem., 1992, 96:9944) disclose that the higher activity and C2xcx9cC4 olefins selectivities was observed over an activated carbon supported iron based catalyst promoted by manganese and potassium promoters.
More recently, U.S. Pat. Nos. 6,274,029, 5,689,031, 5,378,348 disclose diesel fuels or blending stocks having excellent lubricity, oxidative stability and high cetane number are produced from non-shifting Fischer-Tropsch processes by separating the Fischer-Tropsch product into a lighter and heavier fractions, e.g., at about 370xc2x0 C., subjecting the 370xc2x0 C.+ fraction to hydro-treating, and combining the 370xc2x0 C.+ portion of the hydrotreated product with the lighter fraction that has not been hydrotreated.
The production of clean, high cetane number distillates from Fischer-Tropsch waxes has been discussed in the open literature, but few literatures that reported the process for preparing directly diesel distillate with high quality from synthesis gas over an activated carbon supported cobalt based catalyst have appeared.
This invention will first show that the diesel distillates with sulfur-free, nitrogen-free or aromatics-free can be synthesized from synthesis gas over modified Fischer-Tropsch catalyst that cut off the heavier end of the Schulz-Flory distribution.
In accordance with this invention, a clean distillate useful as a fuel heavier than gasoline, e.g., useful as a diesel fuel or as a diesel fuel blend stock and having cetane number of at least about 60, preferably at least about 70, is produced, preferably direct from synthesis gas through Fischer-Tropsch process over an activated carbon supported cobalt based catalyst. In particular, there is provided a process for preparing directly diesel distillates from synthesis gas over modified Fischer-Tropsch catalysts that cut off the heavier end of the Schultz-Flory distribution.